Kinetics of the tropospheric formaldehyde loss onto mineral dust and urban surfaces

The kinetics of the heterogeneous reaction between gaseous HCHO and TiO2/SiO2 mineral coatings were investigated using a coated-wall flow tube to mimic HCHO loss on mineral aerosol and TiO2 coated depolluting urban surfaces. The measured uptake kinetics were strongly enhanced when the flow tube was irradiated with 340–420 nm UV light with an irradiance of 1.45 mW cm−2. The associated BET uptake coefficients ranged from (3.00 ± 0.45) × 10−9 to (2.26 ± 0.34) × 10−6 and were strongly dependent on HCHO initial concentration, relative humidity, temperature, and TiO2 content in the mineral coating, which ranged from 3.5 to 32.5 ppbv, 6–70%, 278–303 K, and 1–100 %wt, respectively. The measured kinetics were well described using a Langmuir–Hinshelwood type formalism. The estimated uptake coefficients were used to discuss the importance of heterogeneous HCHO surface loss, in terms of deposition lifetimes, as compared to major homogeneous gas-phase losses such as OH reaction and photolysis. It is found that deposition may compete with gas-phase removal of HCHO in a dense urban environment if more than 10% of the urban surface is covered with TiO2 treated material.